The objective of the proposed research is to provide a basis for incorporating the experience of both sexes into population analysis. Because of the difficulties involved in capturing the interactions between the sexes, population models are generally limited to one-sex populations. Thus the "two-sex problem" has emerged as the major unresolved issue in formal demography, and the lack of an acceptable solution hampers analyses and projections of fertility and marriage. There is no doubt that "two-sex effects" can be substantial, but little work has been done to carefully identify and measure those effects. The proposed research will address that issue on two levels. First, two-sex population models will be used to study the effects of an imbalance in the number of males and females on marriage, fertility, and population growth. Second, the experience of the United States will be examined to determine the impact of the "marriage squeeze" phenomenon on U.S. nuptiality between 1959 and 1990, and the significance of "two-sex effects" on current projections of the U.S. population.